The development and subsequent production of hydrocarbon fluids from offshore reservoirs and fields normally necessitates the drilling of one or more wells into the ocean floor. Frequently, the water depth at the well site is such at these locations that it becomes necessary to drill from a floating vessel which is adapted to the purpose. These offshore working sites often embody different and varied floor compositions into which the wells are formed. For example, the sea floor can be unconsolidated and give minimal support. However, it can be a consolidated composition and be very supportive of well equipment.
To make an offshore well operable, a drive pipe approximately thirty inches in diameter is jetted into the sea floor and fastened in place. This procedure is found to be most feasible where the sea floor permits.
As a matter of practicality, this drive pipe is frequently incapable of supporting the necessary drilling equipment and accessories. Such equipment includes blowout preventers, flow control means, well heads, and remotely operable controls. In brief, the sea floor positioned equipment can be both bulky and heavy.
To assure an adequate support base for the required operating equipment, the drive pipe is provided with an artificial foundation or support base. In instances where the ocean floor is particularly soft, the foundation or support base will take the form of what is commonly referred to in the drilling industry as a mud mat. The latter comprises basically a large plate which rests on the ocean floor and which functions to distribute the weight of the supported equipment over a large area.
A desirable expedient practiced by the industry when drilling an offshore well from a vessel, is to utilize guidelines which extend between the vessel and the subsea well head. The lines take the form of steel cables, normally four, which attach to both the vessel and to the foundation structure. This arrangement permits equipment to be readily lowered from the surface, guided along the lines, and thus to be properly aligned for engagement with equipment at the ocean floor.
It is the usual practice when lowering this form of well apparatus to the ocean floor, to weld first the artificial foundation or support base to the drive pipe. The entire assembly is then jetted into the ocean floor. With the guidelines attached to the foundation member, and extending to the surface, the system is operable to accurately and safely guide a drilling guide base or blowout preventers to communicably engage the conductor.
The common practice of welding the foundation member to the drive pipe at the drilling site, constitutes a relatively expensive operation. This is due to the necessary positioning of the two components, and the necessary welding thereof to form them into a unit.
It can be appreciated that in any such offshore operations, it is desirable to salvage and retain subsea equipment for further use after it has exhausted its use at a particular drill site. However, federal or state regulations usually require that when a subsea well is no longer usable, its use be terminated by severing the conductor below the sea floor a sufficient distance to preclude its being a navigational hazard. However, with the foundation member firmly welded to the drive pipe, it is difficult to salvage either member without inflicting damage that might render them unusable or unsafe.